Fastify Middlie Authentication Bypass Vulnerability (CVE-2026-6270)

Fastify is a fast and low overhead web framework for Node.js. @fastify/middie is a middleware engine for Fastify. A critical vulnerability, CVE-2026-6270, exists in @fastify/middie versions 9.3.1 and earlier. This flaw stems from the failure to register inherited middleware directly on child plugin engine instances. Specifically, when authentication middleware is established in a parent scope of a Fastify application, any subsequent child plugins utilizing @fastify/middie will fail to inherit this middleware. Consequently, routes defined within these child plugin scopes become susceptible to unauthenticated requests, effectively bypassing intended authentication and authorization mechanisms. The recommended remediation is to upgrade to @fastify/middie version 9.3.2, which addresses this vulnerability.

Attack Chain

1. An attacker identifies a Fastify application using @fastify/middie versions 9.3.1 or earlier.
2. The application has authentication middleware registered in a parent scope to protect certain routes.
3. The attacker discovers a child plugin registered with @fastify/middie, which is intended to inherit the parent's authentication middleware.
4. Due to the vulnerability, the child plugin does not properly inherit the authentication middleware.
5. The attacker crafts a request to a route within the vulnerable child plugin.
6. The request bypasses the authentication checks that were intended to protect the route.
7. The server processes the unauthenticated request and potentially exposes sensitive data or functionality.
8. The attacker successfully accesses protected resources without proper authorization, potentially leading to data breaches or unauthorized actions.

Impact

Successful exploitation of CVE-2026-6270 allows attackers to bypass authentication mechanisms in Fastify applications using vulnerable versions of @fastify/middie. This can lead to unauthorized access to sensitive data, modification of application settings, or execution of privileged actions. The CVSS v3.1 base score for this vulnerability is 9.1, indicating a critical severity level. Organizations using affected versions of @fastify/middie are at risk of data breaches, compliance violations, and reputational damage if this vulnerability is exploited.

Recommendation

• Immediately upgrade to @fastify/middie version 9.3.2 to remediate CVE-2026-6270 (reference: Overview section).
• Deploy the Sigma rule to identify potential exploitation attempts targeting vulnerable Fastify applications (reference: rule "Detect Unauthenticated Access to Fastify Child Routes").
• Review Fastify application configurations to identify instances where authentication middleware is intended to be inherited by child plugins using @fastify/middie (reference: Overview section).